Reflex klystron-type electron tube



April 7,1959 W.-VElTH REF LEX KLYSTRON-TYPE ELECTRON TUBE Filed Nov. 24,1954 United States Patent REFLEX KLYSTRON-TYPE ELECTRON TUBE WernerVeith, Munich, Germany, assignor to Siemens & Halske Aktiengesellschaft,Munich and Berlin, Germany, a German corporation Application November24, 1954, Serial No. 470,913 Claims priority, application GermanyDecember 23, 1953 Claims. (Cl. 315-519) This invention is concerned withan electron tube for very short waves, especially a tube of the reflexklystron type in which an electron reversal is effected in the spacefacing a reflection electrode.

The invention is particularly important for reflex klystrons and othersimilar tubes in which is obtained anelectron reversal. In theconstruction of eflicient reflex klystrons, care is usually taken tokeep the plate voltage as low as possible while using a current as highas possible. The increase in the efliciency that may thereby be obtainedis, however, limited by two conditions, namely, first by the yield ofthe tube and, second, by the effect of the space charge in the reflectorarea.

With constant current, the space charge, as is known, increases steadilywith dropping voltage. At the reversal area of the electrons, facing thereflector plate, the space charge becomes theoretically infinitely greatand the electrons scatter laterally instead of returning to the gridspace in the direction of the injection path. Attempts have been made toprevent this spreading apart at the reflector plate by giving it and itsvicinity suitable configuration.

The invention points another way, namely, the provision of means forconsiderably reducing the space charge in the reversal space. Thespreading apart is in accordance with the invention not prevented; onthe contrary, the space charge in the reversal space is appreciablyreduced, for example, by the provision of a particularly large reflectorsurface. The invention accordingly deviates from the usual reflexklystrons in which the electrons move in a certain direction and permitsthe electrons to diverge ray-like in all directions. With high currentdensities, there is, due to the high space charge at any rate even inthe nondelayed portion of the path a great outwardly directed force, andthis force is preferably utilized for the production of the divergentray beam.

The invention will now be explained with reference to the accompanyingvery much simplified drawings showing two embodiments each illustratingonly the parts which are essential for the understanding of thestructure.

Fig. 1 shows an arrangement comprising an anode and a large area concavecathode 1 with relatively low current density, which is in known manneremployed so as to produce at a point Within the anode space a narrowbeam. This beam diverges upon passing through the anode 10 and throughthe gap formed by grid electrodes 4 and 5 which are connected with walls2 and 3 of the resonator and thereupon spreads strongly due to thepreviously described space charge forces. A concave plate is providedwhich serves as a reflector. The electron motion takes place within thebeam indicated by the dotted lines 7 and 8.

Fig. 2 illustrates another embodiment. Instead of using a large-areacathode, there is provided a crosssectionally relatively smallhigh-capacity cathode indicated at 9. This cathode may, for example, bea capillary metal cathode with high electron yield, which is disposed2,881,350 Patented Apr. 7, 1959 so close to the anode 10 that very highcurrent values can be produced at relatively low voltage. The result isagain a, strongly diverging ray beam. The remaining numerals in Fig. 2indicate parts corresponding to those indicated by like numerals in Fig.l.

The following aspects may be explained more. particularly as beingessential for the operation of the reflex klystron according to theinvention. The formation, in the reflector space, of a virtual cathodemust be prevented. It was shown experimentally and theoretically, inthe, case of. the formation of a virtual cathode, that the returningelectrons are so far as their velocity is concerned only veryindistinctly defined, such fact being reflected in particularly reducedsteepness of the collector current-collector voltage characteristic.This steepness represents a criterion for the quality of the phasefocusing of a reflex klystron, because it signifies that only fewelectrons of a certain velocity group return to the grid space and thatthe initiation of the oscill'ation with a certain frequency is possibleonly with a low degree of efiicieucy. Contrariwise, great steepnesssignifies the initiation of a certain frequency with a high degree ofefliciency.

The two grids of the oscillation circuit are in the two figures concavewith respect to the entering primary ray and are preferably not disposeddirectly at the narrowest area of the beam, so that the arrangement mayhave a relatively great grid capacitance. This arrangement causes adeterioration of the circuit quality, which is however compensated bythe increased current and by the conditions in the reflector space whichare improved by the invention.

The reflex klystrons according to the invention are due to theirrelatively great band width particularly suitable for modulationpurposes.

The strict condition concerning the nonappearance of a virtual cathodeis theoretically accurately known. Such condition constitutes alimitation for the length of the passage in the reflector space and maybe mathematically formulated such that the length becomes l/V of thespace between the cathode and the anode-all factors calculated in termsof a plane arrangement. Favorable data accordingly are obtained for thearrangement by applying the following simplified formula, namely whereinF is the effective reflector surface, K the cathode surface, a thespacing of the reflector area from the preceding grid electrode and kthe cathode-anode spacing. It will be seen from the formula andreferring to the figures, that the ratio of the effective reflectionsurface (F) to the cathode surface (K) is greater than twice the ratioof the square of the suppression path (a) preceding the reflector andthe acceleration path (k).

Changes may be made within the scope and spirit of the appended claimsin which is defined what is believed to be new and desired to haveprotected by Letters Patent.

I claim:

1. In a reflex klystron-type electron tube for very short waves, havinga cathode and an anode spaced therefrom and defining an electronacceleration space therewith and having grid means spaced from saidanode and forming a gap, and having a reflector forming a reflectionsurface spaced from said grid means and defining therewith an entirelyunobstructed electron reflection space, a resonator disposed entirelyoutside said reflection space and connected with said grid means, theelectron beam emanating from said cathode progressively diverging alongits passage from said anode and across said gap and thereupon stronglyspreading on its passage across said reflection space. to saidreflection surface, electron reversal being effected within saidreflection space, the ratio of the effective reflection surface of saidreflector to the cathode surface being greater than twice the ratio ofthe square of the suppression path across said reflection space and theacceleration ing across said acceleration space.

2. In an electron tube for very short waves operating in the manner of areflex klystron and having a cathode and an anode spaced therefrom anddefining an electron acceleration space therewith and having grid meansspaced from said anode and mutually spaced by a gap, and having aconcave reflector forming a relatively largearea reflection surfacespaced from said grid means and defining an entirely unobstructedelectron reflection space therewith, said reflector operating in themanner of a reflex electrode, a resonator disposed entirely outside saidreflection space and connected with said grid means, said grid meanscontrolling the electron beam emanating from said cathode to divergeprogressively along its passage from said anode and across the gaptherebetween, thereby causing said beam to spread strongly on itspassage across said electron reflection space to said reflecpath extendvtion surface, electron reversal being efiected within said reflectionspace, the ratio of the effective reflection surface of said reflectorto the'jcathode surface being greater than twice the ratio of the squareof the suppression path extending across said reflection space and theacceleration path extending across said acceleration space for thepurpose of considerably decreasing the space charge within said electronreversal space.

3. The structure defined in claim 2, comprising a concave relativelylarge-area cathode, said cathode being disposed relatively remote fromsaid anode.

4. The structure defined in claim 2, comprising a crosssectionallyrelatively small high-capacity cathode disposed relatively close to saidanode.

5. The structure defined in claim 2, comprising concave grid meansdisposed in the path of said divergent electron beam relativelyconsiderably spaced from said anode.

References Cited in the file of this patent UNITED STATES PATENTS2,460,332 Bowman-Manifold Feb. 1, 1949 2,513,371 Shepherd July 4, 19502,659,024 Bernier et a1. Nov. 10, 1953 FOREIGN PATENTS 7 602,796 GreatBritain June 3, 1948 665,505 Great Britain Jan. 23, 1952

